Erbium doped optical fiber amplifier for automatically tracing and filtering wavelength of transmitted light and its operation method

ABSTRACT

An erbium doped fiber amplifier, which is equipped with an optical filter at its output port to eliminate noise caused by properties of the amplifier, automatically traces and filters transmitted light signal wavelengths using a wavelength control unit for adjusting the central wavelength of the optical filter to correspond to the wavelength of the transmitted light signal after determining the wavelength of the transmitted light signal.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C. §119 from an applicationfor ERBIUM DOPED OPTICAL FIBER AMPLIFIER FOR AUTOMATICALLY TRACING ANDFILTERING WAVELENGTH OF TRANSMITTED LIGHT AND ITS OPERATION METHODearlier filed in the Korean Industrial Property Office on the 1^(st) dayof Aug. 1996 and there duly assigned Ser. No. 1996-32235, a copy ofwhich application is annexed hereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical amplifier, and moreparticularly, it relates to an erbium doped fiber amplifier (EDFA) andits method of operation. The EDFA provided by this inventionautomatically traces transmitted light wavelengths and adjustablyfilters the wavelengths of light signals to be transmitted by adjustingthe central wavelength of an optical filter installed at the outputterminal of the amplifier, thereby preventing propagation of noisecaused by the properties of the optical amplifier.

2. Description of the Related Art

When a transmission terminal in an optical communication networkconverts an electric signal into a light signal, and transmits it to adesired destination using optical fiber, EDFA are usually used toamplify the weakened light signals at predetermined distances along thetransit route. This practice of periodic re-amplification ensures thetransmission of stable signals. Such amplifiers are also typicallyinstalled in reception and transmission terminals to amplify electricpower and perform pre-amplification.

An EDFA commonly includes a tunable filter that removes noise introducedinto the amplified light signal during the amplification process. Such afilter has a central wavelength at which a light signal passed throughthe filter receives the least attenuation. Signal components atdifferent wavelengths receive greater attenuation the farther thosewavelengths are from the filter's central wavelength. When the filter istuned for a central wavelength equal to the nominal wavelength of anincoming light signal, noise introduced by the amplifier can beefficiently eliminated from the amplified signal.

However, light signals received over optical communication networks donot always meet nominal parameters. In practice, such a signal will varyaccording to characteristics of the various components of the opticalamplification device that generated or boosted it. Thus, transmittedlight signal wavelengths may change as the optical amplification deviceoperates over an extended period of time. They can also be influenced byambient temperatures. To compensate for these effects, awave-length-fixing type filter or a convertible-to-manual filter may beutilized. However, these approaches generally result in a loss of signalstrength when instantaneous changes occur in the wavelength of thereceived signal. Also, they potentially create problems by decreasingthe intensity of the light signals generated by the amplificationdevice.

The use of tunable filters to remove noise introduced into opticalcommunications signals by optical amplifiers is well known in theliterature. For example, U.S. Pat. No. 4,945,531 provides a system withseveral tunable filters interposed between an optical wavelengthdemultiplexer and a multiplexer to filter spontaneous emission noisefrom around several channels in a WDM communications signal. U.S. Pat.No. 5,644,423 also discloses an amplifier for WDM signals thatautomatically adjusts a tunable filter central wavelength to trace thewavelength of a selected channel for gain control. This latter systemconstitutes a significant advance in EDFA filtering technology, but itrequires a complicated wavelength feedback system that includes areference oscillator and a synchronous detector to generate a wavelengtherror signal.

U.S. Pat. No. 5,570,221, entitled "Light Amplification Device" andissued Oct. 29, 1996 to Fujita, the disclosure of which is incorporatedherein by reference, provides another substantial advance in filteringcontrol technology. This patent discloses an EDFA that automaticallyadjusts the central wavelength of a tunable filter, positioned downbeamfrom a doped fiber amplification element, to trace the wavelength of theinput signal component of an amplified signal.

The device provides many advantages, but it has a complex structure andrelies upon specific features found in the output signals of typicalcurrent fiber amplifiers to achieve its objectives. In particular, ituses a two level wavelength sweep procedure: in a broad sweep, it locksonto a desired wavelength in the amplified signal by finding thewavelength where a negative peak occurs in the second derivative of theamplified signal intensity. In a narrow sweep, the intensity peak lockedfrom the broad sweep is traced by repeatedly sweeping a narrow band tofind the wavelength therein at which the first derivative of theintensity vanishes. This approach has undeniable elegance, but itrequires both first and second order differentiators and also reliesupon the amplified signal having a well-defined, single-peak featurecorresponding to the input light signal. If the input signal isdegraded, then the tests this system uses in its double sweep proceduremay not provide reliable tracing results.

U.S. Pat. No. 5,572,351, entitled "Optical Communications Systems" andissued Nov. 5, 1996 1996 to Hadjifotiou, the disclosure of which isincorporated herein by reference, also provides an advanced wavelengthtracing system for an EDFA having a tunable filter. The disclosed systemautomatically adjusts the central wavelength of the tunable filter byadding a pilot signal to the data signal at a wavelength spaced apart(above or below) the band occupied by the data signal. The frequency ofthe pilot component of the received signal is detected by the EDFA andthe central frequency of the data band in the received signal is deducedfrom the received pilot frequency. This ingenious system also hascertain limitations, such as requiring use of a pilot signal and relyingupon comparable dispersion in the pilot signal and the data signal.

These systems, while providing significant advances, have certainlimitations from which I have concluded that a wavelength-controlledEDFA with further improvements is needed. Such a system should providerobust wavelength tracing control without requiring special signaltransmission formats. It should utilize only thoroughly proven controlsystem components and should not rely upon specialized optical devicesfor operation. It also should not rely upon specific input signalfeatures that may not be attainable in suboptimal operating situations.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide an erbium dopedfiber amplifier (EDFA) and its operation method for automaticallytracing and filtering the transmitted light wavelengths in order toadjust the central wavelength of an optical filter, installed in itsoutput stage, to the wavelength of the transmitted light, the amplifierbeing equipped with a microprocessor.

To this and other objectives, the present invention provides in a firstaspect an optical amplifier, comprising an optical amplifying unithaving an input port and an output port, an optical filter coupled tothe output port and having an adjustable central wavelength, and awavelength control unit in communication with the optical filter. Theoptical amplifying unit receives an incoming light signal at its inputport and emits an amplified outgoing light signal at its output port.The optical filter receives the outgoing light signal and removes fromit a noise component introduced by the optical amplifying unit.

The wavelength control unit includes an intensity detector for detectingan intensity of the outgoing light signal at each one of a plurality ofdiscrete control levels within a control level range; an intensitycomparator for comparing a first intensity, detected at a first one ofthe plurality of control levels, to a second intensity signalrepresentative of a second intensity detected at a second one of theplurality of control levels; and a storage unit for storing a valuerepresentative of the first intensity when the first intensity is notless than the second intensity and storing a value representative of thesecond intensity when the first intensity is less than the secondintensity.

In a second aspect, the present invention provides a method forautomatically adjusting a central wavelength of an optical filter totrace a peak intensity wavelength of an output light signal. The methodcomprises the step of adjusting the central wavelength in accordancewith each one of a plurality of discrete control levels, with a nextlower control level corresponding to each one of the plurality ofcontrol levels. It comprises a further step of measuring an intensity ofthe output light signal at each one of the plurality of control levelsand storing a value representative of the intensity. It also includesthe step of selecting as a maximum intensity control level one of theplurality of control levels corresponding to a maximum value of aplurality of values consisting of the value representative of theintensity at each one of the plurality of control levels.

The method of the present invention further includes the step ofadjusting the central wavelength in accordance with the next lowercontrol level corresponding to the maximum intensity control level. Itincludes a step of measuring a next intensity of the output light signalat the next lower control level corresponding to the maximum intensitycontrol level. It also includes the steps of comparing a valuerepresentative of the next intensity to the maximum value and generatinga comparison result and adjusting the central wavelength in accordancewith the comparison result.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete appreciation of this invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanying drawingfigures, in which like reference symbols indicate the same or similarcomponents, wherein:

FIG. 1 is a block diagram of an existing single pumped doped fiberoptical amplifier;

FIG. 2 is a graph showing the elimination of noise from the amplifierusing a conventional optical filter;

FIG. 3 is a block diagram of an EDFA which automatically traces andfilters the wavelengths of the transmitted light signal according to thepresent invention;

FIG. 4 is a graph showing how the intensity of the output light changesaccording to the central wavelength of an optical filter, which in turncorresponds to the intensity of the transmitted light, according to thepresent invention; and

FIGS. 5A and 5B provide a flow chart illustrating the method ofoperation of the present invention in automatically tracing andfiltering the wavelengths of the transmitted light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a block diagram of an existing single pumped amplifier thatprovides an example of the general context of the present invention. Aninput connector connects an optical fiber, leading from outside theamplifier, to an internal optical fiber contained in the amplifier. Aseparation tap 2 splits the input light signal from the connectedoptical fiber into an incoming light signal and an input side monitoringlight signal in accordance with a predetermined ratio and sends them toan optical isolator 4 and an input side photodiode 12, respectively.Photodiode 12 measures the intensity of the incoming light signal.

Optical isolator 4, which has one input terminal and one outputterminal, applies only minimal attenuation to light signals propagatingfrom the input terminal to the output terminal, but it effectivelyinterrupts light signals travelling toward the input terminal from theoutput terminal. Optical isolator 4 thus prevents distortion of theincoming light signal by interrupting feedback caused by amplifiedspontaneous emissions (ASEs) generated by amplification components ofthe amplifier (such as light-amplifying optical fibers).

The incoming light signal proceeds from optical isolator 4 to awavelength division multiplexer (WDM) 6. WDM 6 receives two differentlight signals at different wavelengths through its respective inputterminals and combines them into a single multiplexed signal, which itsends out through an optical fiber terminal. The incoming(communication) light signal has a wavelength of 1,550 nm, whereas apump laser diode, used as an excitation light source, provides a powersignal with a wavelength of typically 980 nm or 1,480 nm. WDM 6 sendsthe power signal (wavelength, e.g., 980 nm) and the communication signal(wavelength 1,550 nm) through an output terminal to an erbium dopedamplification fiber (EDF) 16.

EDF 16 is an optical fiber doped with the rare-earth metal erbium(element number 68), which provides the fiber with high absorption ratesat specific wavelengths such as 800, 980, and 1,480 nm. This doped fiberabsorbs the power signal and thereby amplifies the communication signal,which has a spectral bandwidth of about 60 nm centered at apredetermined wavelength (e.g., 1,550 nm). The output end of EDF 16 isconnected to an optical isolator 8, which in turn is connected to aseparation tap 10. Optical isolator 8 interrupts light signalsreflecting back from separation tap 10 or other optical devices in thepropagating light signal's downbeam path. Finally, separation tap 10 isconnected to the output stage fiber by an output connector.

Separation tap 10 receives an outgoing light signal from opticalisolator 8 and splits it into an output light signal, to be output tothe downbeam fiber connected via the output connector, and an outputside monitoring light signal for monitoring the output light signal. Theoutput side monitoring light signal is received by an output sidephotodiode 14. Photodiode 12 generates an (electrical) input monitoringsignal from the input side monitoring light signal, and photodiode 14generates an (electrical) output monitoring signal. These two monitoringsignals are amplified, respectively, by two analog amplifiers 20 and 22.An electronic controller 24 receives the amplified monitoring signalsand, in accordance with them, controls the output of a pump laser diode18.

FIG. 2 illustrates an inherent problem that arises in EDFAs when thepower signal generated by pump laser diode 18 is used with EDF 16 toamplify the incoming light signal. On the one hand, fiber opticcommunication over long distances requires periodic amplification of thecommunication signals, due to signal attenuation from the transmissionfibers and other optical elements through which the signals propagate.On the other hand, to realize the potentially large data bandwidthsafforded by fiber optic systems, received signals must have relativelynarrow spectral bandwidths. The input light signal received by an EDFAmay have a desirably narrow signal peak, such as peak 28 in FIG. 2. Butthe amplification process of EDF 16 introduces into the amplified signala substantial noise component 29. Accumulation of such noise componentsover several amplification stages would unacceptably deteriorate thecommunication signal peak.

To address this problem, the EDFA includes a wavelength variation filter26 (equivalently, a wavelength fixing filter) that receives the outputlight signal from separation tap 10 filters out noise introduced duringthe amplification process. When the central wavelength of filter 26 isset at the central wavelength of the input light signal, e.g., at 1,550nm, the noise component 29 can be efficiently eliminated from theamplified signal, leaving a sharpened signal 30.

FIG. 3 is a block diagram of an EDFA in accordance with the presentinvention. This EDFA includes separation taps 202 and 214, opticalisolators 204 and 208, a wavelength division multiplexer (WDM) 206, anEDF 210, a tunable optical filter 212, photodiodes 216 and 220, and apump laser diode 218 whose features and functions are the same as thecorresponding components of the optical amplifier illustrated in FIG. 1.Detailed description of these components will therefore be omitted.

In the present invention, a microprocessor 274 replaces electroniccontroller 24 of the existing optical amplifier for controlling theoutput of pump laser diode 18. A/D converters 222 and 228 are interposedbetween microprocessor 224 and photodiodes 216 and 220 and convert theanalog monitoring signals generated by photodiodes 216 and 220 intocorresponding digital monitoring signals. Microprocessor 224 receivesmeasurements of the incoming and outgoing light signal intensities, asdetected by photodiodes 216 and 220 and converted by A/D converters 222and 228, and thereby controls the output power of pump laser diode 218.

Microprocessor 224 serves in part as an intensity comparator of awavelength control unit by receiving signals representing the measuredintensity of the outgoing light signal and generating control signals toadjust the central wavelength of optical filter 212. The control signalsfrom microprocessor 224 drive the central wavelength to a wavelengththat induces a maximum intensity in the output light signal. Opticalfilter 212 is installed between output side optical isolator 208 andseparation tap 214. D/A converter 226 is installed between opticalfilter 212 and microprocessor 224 to convert the digital signals frommicroprocessor 224 into analog signals.

FIG. 4 illustrates the attenuation of the output light signal due tofilter 212 as a function of the filter's central wavelength. When theoutput light signal has a peak intensity at 1,550 nm, its total lightintensity is greatest when the central wavelength of the optical filteris adjusted to 1,550 nm, matching the signal's peak intensitywavelength. As the central wavelength of the filter increases ordecreases, the total intensity of the output light signal decreasesprecipitously. For example, the graphs shown in FIG. 4 indicate thatplacing the central wavelength of the filter 0.5 nm below the outputsignal's peak intensity wavelength reduces the peak intensity by about 1dBm and the total intensity of the output signal by about 5 dBm. Largerdiscrepancies between the central wavelength and the peak intensitywavelength result in greater attenuation, and the attenuation rateincreases as the discrepancy increases. Thus, effective use of theavailable output of pump laser diode 218 depends critically uponcontrolling the filter central wavelength to track the peak intensitywavelength of the output light signal.

FIGS. 5a and 5b illustrate a method of operation for the device shown inFIG. 3. First, at step 502, microprocessor 224 initializes the system.This entails configuring various software registers, as will be apparentto persons skilled in the art, and specifying several operatingparameters that will be described hereinbelow. Microprocessor 224 thensets a control level value (denoted Vhex) for optical filter 212 at step504, based on the nominal wavelength of the input light signal. Vhexdetermines the central wavelength setting for optical filter 212. As theoperation method proceeds, Vhex will be varied over a fixed rangedetermined by the word length (number of bits) accommodated by D/Aconverter 226, as will be understood by persons of skill in the digitalcontrol art. In terms of wavelength ranges, the range of the controllevel value (Vhex) will usually be from 1,540 nm to 1,560 nm for anominal wavelength of 1,550 nm.

Microprocessor 224 controls the central wavelength of optical filter 212at step 506 by successively setting the control level value at each ofseveral levels into which the range for Vhex is divided, with the numberof levels depending upon the tuning resolution of optical filter 212 andupon the word length of D/A converter 226. Starting with a first levelVhex+1, at step 508 microprocessor 224 measures and stores the intensityvalue of the output light signal. This value is stored in a storagedevice (not shown), which may be a RAM device and which may constitute acomponent part of microprocessor 224. The intensity of the output lightis measured by output side photodiode 216, and sent to microprocessor224 through A/D converter 228.

At the step 506 microprocessor 224 determines whether the current levelis the last level in the range for Vhex, which is identified for futurereference in the initialization step 502. If the current level is notthe final level, then microprocessor 224 performs another iteration ofsteps 506 and 508. If the current level is the last level, then at step512 microprocessor 224 determines whether the maximum value of theoutput light intensity is within fixed control level limits. The controllevel limits may be chosen within a range of from +3 dBm to -35 dBm andwill depend upon the characteristics of the amplifier components. Thecontrol level limits are set at the initialization step 502. If themaximum measured intensity does not fall within the control levellimits, then the process returns to step 504 to reset Vhex of opticalfilter 212 in accordance with the value held in A/D converter 228.

If the maximum intensity value of the output light does fall within thecontrol level limits, then at step 514 (FIG. 5B) microprocessor 224enters a detailed trace mode for controlling the central wavelength ofoptical filter 212. At step 516 of the detailed trace mode,microprocessor 224 adjusts the central wavelength of optical filter 212to the control level that is one level below the control level at whichthe maximum intensity value of the output light signal was previouslymeasured, and the intensity level is again measured. microprocessor 224increases the control level of the central wavelength of optical filter212 by one level at step 518 and once again measures the intensity ofthe output light signal. At step 520, microprocessor 224 compares theoutput light intensity measured at step 518 to the output lightintensity measured at step 516. Thus, the intensity at the control levelof the previously measured maximum intensity is compared with theintensity at a control level one level lower.

If the output light intensity measured at step 518 is not less than theoutput light intensity measured at step 516, then the process returns tostep 518 where the operations of increasing the control level andmeasuring the output light intensity are repeated. If the output lightintensity measured at step 518 is less than the output light intensitymeasured at step 516, then at step 522 microprocessor 224 decreases thecontrol level by one level and measures the intensity of the outputlight. Microprocessor 224 then determines at step 524 whether the outputlight intensity after decreasing the control level value by one level isless than the intensity before decreasing the control level. If theoutput light intensity before decreasing the level value is less thanthe intensity after decreasing the control level, then the processreturns to the step 522 to repeat the operations of reducing the controllevel and measuring the output light intensity.

If the output light intensity before decreasing the control level isgreater than the intensity after decreasing the control level, then atstep 526 microprocessor 224 determines whether a subtracted intensityvalue, obtained by subtracting the output light intensity measured atthe current control level from the maximum output light intensity storedpreviously, exceeds an effective range. The effective range, set at theinitialization step 502, is usually about 5 dBm and is chosen inaccordance with the resolution provided by the word length of D/Aconverter 226. If the subtracted intensity value exceeds the effectiverange, then the process returns to step 504, where the control levelvalue of optical filter 212 is reset in accordance with the wavelengthof the transmitted light as determined by the value held by A/Dconverter 228, and the process repeats. If the subtracted intensityvalue is less than the effective range, then the central wavelength ofoptical filter 212 is adjusted to the current control level. The processthen goes returns to the step 518 for continuing micro-control of thecentral wavelength of optical filter 212 in accordance with thewavelengths of subsequently received input light signals.

As illustrated, the present invention provides a device and method fortracing the central wavelength of an amplified light signal by adjustingthe central wavelength of an optical filter, installed in the outputport, to a wavelength providing maximum output light signal intensity.The present invention therefore provides an optical amplifier withsubstantially improved reliability and efficiency. It should be notedthat the device and method of the present invention achieve theiradvantageous results by providing an output light signal with highintensity, relative to the available power from the excitation lightsource, while maintaining the required sharpness of the output signal.

It should be understood also that the present invention is limitedneither to the particular embodiment disclosed herein as the best modecontemplated for carrying out the invention, nor to the specificembodiments described in this specification, but instead fullyencompasses the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. An optical amplifier, comprising:an opticalamplifying unit, having an input port and an output port, for receivingan incoming light signal at said input port and emitting an amplifiedoutgoing light signal at said output port; an optical filter, coupled tosaid output port and having an adjustable central wavelength, forreceiving said outgoing light signal and removing therefrom a noisecomponent introduced by said optical amplifying unit; and a wavelengthcontrol unit in communication with said optical filter, said wavelengthcontrol unit includingan intensity detector for detecting an intensityof said outgoing light signal at each one of a plurality of discretecontrol levels within a control level range, an intensity comparator forcomparing a first intensity signal representative of a first intensitydetected at a first one of said plurality of control levels to a secondintensity signal representative of a second intensity detected at asecond one of said plurality of control levels, and a storage unit forstoring a value representative of said first intensity when said firstintensity is not less than said second intensity and storing a valuerepresentative of said second intensity when said first intensity isless than said second intensity.
 2. The optical amplifier of claim 1,further comprising:an input side separation tap, coupled to said inputport, for splitting an input light signal into said incoming lightsignal and an input side monitoring light signal in accordance with afirst predetermined ratio; and an input side photodiode, coupled to saidinput side separation tap, for receiving said input side monitoringlight signal and generating therefrom an input monitoring signal.
 3. Theoptical amplifier of claim 2, wherein said intensity detectorincludes:an output side separation tap, coupled to said output port, forreceiving said outgoing light signal and splitting said outgoing lightsignal into an output side monitoring light signal and an output lightsignal in accordance with a second predetermined ratio; and an outputside photodiode, coupled to said output side separation tap and incommunication with said intensity comparator, for receiving said outputside monitoring light signal and generating an output monitoring signalrepresentative of said intensity of said outgoing signal.
 4. The opticalamplifier of claim 1, further comprising an input side A/D converter, incommunication with said input side photodiode, for converting said inputmonitoring signal into a digital input monitoring signal.
 5. The opticalamplifier of claim 3, further comprising an input side A/D converter, incommunication with said input side photodiode, for converting said inputmonitoring signal into a digital input monitoring signal.
 6. The opticalamplifier of claim 1, wherein said first and second intensity signalsare digital signals and said intensity detector includes an output sideA/D converter, in communication with said intensity comparator, forconverting a first output monitoring signal representative of said firstintensity into said first intensity signal and a second outputmonitoring signal representative of said second intensity into saidsecond intensity signal.
 7. The optical amplifier of claim 3, whereinsaid first and second intensity signals are digital signals and saidintensity detector includes an output side A/D converter, incommunication with said output said photodiode and said intensitycomparator, for converting a first output monitoring signalrepresentative of said first intensity into said first intensity signaland a second output monitoring signal representative of said secondintensity into said second intensity signal.
 8. The optical amplifier ofclaim 1, wherein said wavelength control unit further includes a D/Aconverter, in communication with said optical filter, for converting adigital control signal into an analog control signal and providing saidanalog control signal to said optical filter.
 9. The optical amplifierof claim 3, wherein said wavelength control unit further includes a D/Aconverter, in communication with said optical filter, for converting adigital control signal into an analog control signal and providing saidanalog control signal to said optical filter.
 10. The optical amplifierof claim 3, wherein said optical amplifying unit includes:a pump laserdiode for generating a power signal; a wavelength division multiplexer,coupled to said input side separation tap and said pump laser diode, forreceiving said power signal from said pump laser diode and said incominglight signal from said input port and outputting a multiplexed lightsignal containing said power signal and said incoming signal; a rareearth doped optical fiber, coupled to said wavelength divisionmultiplexer, for receiving said multiplexed light signal and generatingtherefrom said outgoing light signal; a first optical isolator,interposed between said input side separation tap and said wavelengthdivision multiplexer, for interrupting reflected signals; and a secondoptical isolator, interposed between said rare earth doped optical fiberand said optical filter, for interrupting reflected signals.
 11. Amethod for automatically adjusting a central wavelength of an opticalfilter to trace a peak intensity wavelength of an output light signal,said method comprising the steps of:adjusting said central wavelength inaccordance with each one of a plurality of discrete control levels, witha next lower control level corresponding to each one of said pluralityof control levels; measuring an intensity of said output light signal ateach one of said plurality of control levels and storing a valuerepresentative of said intensity; selecting as a maximum intensitycontrol level one of said plurality of control levels corresponding to amaximum value of a plurality of values consisting of said valuerepresentative of said intensity at each one of said plurality ofcontrol levels; adjusting said central wavelength in accordance withsaid next lower control level corresponding to said maximum intensitycontrol level; measuring a next intensity of said output light signal atsaid next lower control level corresponding to said maximum intensitycontrol level; and comparing a value representative of said nextintensity to said maximum value and generating a comparison result; andadjusting said central wavelength in accordance with said comparisonresult.